1. Field of the Invention
The present invention relates to an image processing apparatus, image processing method and computer program, which read images on both sides of a document and prevent showthrough in which the printing on the rear side is visible through the paper.
2. Description of the Related Art
Conventionally, an image reader, typified by a scanner, fax machine, and copying machine can automatically acquire an front side image and a rear side image of a document using an ADF (Auto Document Feeder) and the like at the time when reading out the images from a document. Thus, a user need not place the document printed on both sides on a document glass twice, the front side and the rear side each, so that the load of acquiring the document printed on both sides is reduced. In addition, recently, it has become possible to acquire the front side image and rear side image of a document simultaneously through a seeming single reading operation by preparing two image sensors for reading the front side and rear side each within a single reader.
However, when the conventional image reader reads a document printed on both sides, the rear side image sometimes appears in the front side image depending on the thickness of the document paper or on the amount of light to the image sensors. This constitutes a primary factor of deteriorating the quality of the images read.
Conventionally, various trials have been made concerning the showthrough image problem of the document. For example, such processing is typical that eliminates the rear side image effect from the front side image by subtracting a mirror image of the rear side image from the front side image.
According to Japanese Patent Laid-Open No. H08-265563 (1996), the showthrough is reduced by the processing that eliminates the effect of the rear side image on the front side image by the addition of the front side image and the rear side image.
According to Japanese Patent Laid-Open No. H05-063968 (1993), it determines the background level of a document by prescanning, and eliminates the pixels with the luminance higher than the calculated background level efficiently. In this case, although a contrivance to prevent deterioration in the document color reproducibility is made using formulae, since it does not consider the unexpected appearance of the rear side image, it cannot eliminate the showthrough from the document background having a lot of showthrough.
However, to achieve the foregoing combining processing, the alignment accuracy between the front side image and rear side image of the document is very important. For example, if the registration position between the two sides, that is, the coordinate position of the rear side image corresponding to that of the front side image shifts by 200 μm, this corresponds to a shear of five pixels in the image read at a resolution of 600 dpi. When carrying out subtracting combining of the document with the shear of five pixels, the subtracting combining can sometimes reduce the alignment accuracy on the contrary. Thus, it is essential for the reading device to increase its registration accuracy. When the alignment accuracy is low, showthrough shadows appear as a result of the subtracting combining made on locations where no rear side image appears.
Furthermore, it requires the size of a subtracting combining circuit whose input is two sheets of images, the processing cost and a memory, and these factors present another problem besides the cost for increasing the accuracy of the reading device.